/*******************************************************************************
 * Copyright (c) 2000, 2008 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.eclipse.jface.text;

import org.eclipse.core.runtime.Assert;


/**
 * Implements a gap managing text store. The gap text store relies on the assumption that
 * consecutive changes to a document are co-located. The start of the gap is always moved to the
 * location of the last change.
 * <p>
 * <strong>Performance:</strong> Typing-style changes perform in constant time unless re-allocation
 * becomes necessary. Generally, a change that does not cause re-allocation will cause at most one
 * {@linkplain System#arraycopy(Object, int, Object, int, int) arraycopy} operation of a length of
 * about <var>d</var>, where <var>d</var> is the distance from the previous change. Let <var>a(x)</var>
 * be the algorithmic performance of an <code>arraycopy</code> operation of the length <var>x</var>,
 * then such a change then performs in <i>O(a(x))</i>,
 * {@linkplain #get(int, int) get(int, <var>length</var>)} performs in <i>O(a(length))</i>,
 * {@link #get(int)} in <i>O(1)</i>.
 * <p>
 * How frequently the array needs re-allocation is controlled by the constructor parameters.
 * </p>
 * <p>
 * This class is not intended to be subclassed.
 * </p>
 *
 * @see CopyOnWriteTextStore for a copy-on-write text store wrapper
 * @noextend This class is not intended to be subclassed by clients.
 */
public class GapTextStore implements ITextStore {
	/**
	 * The minimum gap size allocated when re-allocation occurs.
	 * @since 3.3
	 */
	private final int fMinGapSize;
	/**
	 * The maximum gap size allocated when re-allocation occurs.
	 * @since 3.3
	 */
	private final int fMaxGapSize;
	/**
	 * The multiplier to compute the array size from the content length
	 * (1&nbsp;&lt;=&nbsp;fSizeMultiplier&nbsp;&lt;=&nbsp;2).
	 *
	 * @since 3.3
	 */
	private final float fSizeMultiplier;

	/** The store's content */
	private char[] fContent= new char[0];
	/** Starting index of the gap */
	private int fGapStart= 0;
	/** End index of the gap */
	private int fGapEnd= 0;
	/**
	 * The current high water mark. If a change would cause the gap to grow larger than this, the
	 * array is re-allocated.
	 * @since 3.3
	 */
	private int fThreshold= 0;

	/**
	 * Creates a new empty text store using the specified low and high watermarks.
	 *
	 * @param lowWatermark unused - at the lower bound, the array is only resized when the content
	 *        does not fit
	 * @param highWatermark if the gap is ever larger than this, it will automatically be shrunken
	 *        (&gt;=&nbsp;0)
	 * @deprecated use {@link GapTextStore#GapTextStore(int, int, float)} instead
	 */
	@Deprecated
	public GapTextStore(int lowWatermark, int highWatermark) {
		/*
		 * Legacy constructor. The API contract states that highWatermark is the upper bound for the
		 * gap size. Albeit this contract was not previously adhered to, it is now: The allocated
		 * gap size is fixed at half the highWatermark. Since the threshold is always twice the
		 * allocated gap size, the gap will never grow larger than highWatermark. Previously, the
		 * gap size was initialized to highWatermark, causing re-allocation if the content length
		 * shrunk right after allocation. The fixed gap size is now only half of the previous value,
		 * circumventing that problem (there was no API contract specifying the initial gap size).
		 *
		 * The previous implementation did not allow the gap size to become smaller than
		 * lowWatermark, which doesn't make any sense: that area of the gap was simply never ever
		 * used.
		 */
		this(highWatermark / 2, highWatermark / 2, 0f);
	}

	/**
	 * Equivalent to
	 * {@linkplain GapTextStore#GapTextStore(int, int, float) new GapTextStore(256, 4096, 0.1f)}.
	 *
	 * @since 3.3
	 */
	public GapTextStore() {
		this(256, 4096, 0.1f);
	}

	/**
	 * Creates an empty text store that uses re-allocation thresholds relative to the content
	 * length. Re-allocation is controlled by the <em>gap factor</em>, which is the quotient of
	 * the gap size and the array size. Re-allocation occurs if a change causes the gap factor to go
	 * outside <code>[0,&nbsp;maxGapFactor]</code>. When re-allocation occurs, the array is sized
	 * such that the gap factor is <code>0.5 * maxGapFactor</code>. The gap size computed in this
	 * manner is bounded by the <code>minSize</code> and <code>maxSize</code> parameters.
	 * <p>
	 * A <code>maxGapFactor</code> of <code>0</code> creates a text store that never has a gap
	 * at all (if <code>minSize</code> is 0); a <code>maxGapFactor</code> of <code>1</code>
	 * creates a text store that doubles its size with every re-allocation and that never shrinks.
	 * </p>
	 * <p>
	 * The <code>minSize</code> and <code>maxSize</code> parameters are absolute bounds to the
	 * allocated gap size. Use <code>minSize</code> to avoid frequent re-allocation for small
	 * documents. Use <code>maxSize</code> to avoid a huge gap being allocated for large
	 * documents.
	 * </p>
	 *
	 * @param minSize the minimum gap size to allocate (&gt;=&nbsp;0; use 0 for no minimum)
	 * @param maxSize the maximum gap size to allocate (&gt;=&nbsp;minSize; use
	 *        {@link Integer#MAX_VALUE} for no maximum)
	 * @param maxGapFactor is the maximum fraction of the array that is occupied by the gap (<code>0&nbsp;&lt;=&nbsp;maxGapFactor&nbsp;&lt;=&nbsp;1</code>)
	 * @since 3.3
	 */
	public GapTextStore(int minSize, int maxSize, float maxGapFactor) {
		Assert.isLegal(0f <= maxGapFactor && maxGapFactor <= 1f);
		Assert.isLegal(0 <= minSize && minSize <= maxSize);
		fMinGapSize= minSize;
		fMaxGapSize= maxSize;
		fSizeMultiplier= 1 / (1 - maxGapFactor / 2);
	}

	@Override
	public final char get(int offset) {
		if (offset < fGapStart)
			return fContent[offset];

		return fContent[offset + gapSize()];
	}

	@Override
	public final String get(int offset, int length) {
		if (fGapStart <= offset)
			return new String(fContent, offset + gapSize() , length);

		final int end= offset + length;

		if (end <= fGapStart)
			return new String(fContent, offset, length);

		StringBuffer buf= new StringBuffer(length);
		buf.append(fContent, offset, fGapStart - offset);
		buf.append(fContent, fGapEnd, end - fGapStart);
		return buf.toString();
	}

	@Override
	public final int getLength() {
		return fContent.length - gapSize();
	}

	@Override
	public final void set(String text) {
		/*
		 * Moves the gap to the end of the content. There is no sensible prediction of where the
		 * next change will occur, but at least the next change will not trigger re-allocation. This
		 * is especially important when using the GapTextStore within a CopyOnWriteTextStore, where
		 * the GTS is only initialized right before a modification.
		 */
		replace(0, getLength(), text);
	}

	@Override
	public final void replace(int offset, int length, String text) {
		if (text == null) {
			adjustGap(offset, length, 0);
		} else {
			int textLength= text.length();
			adjustGap(offset, length, textLength);
			if (textLength != 0)
				text.getChars(0, textLength, fContent, offset);
		}
	}

	/**
	 * Moves the gap to <code>offset + add</code>, moving any content after
	 * <code>offset + remove</code> behind the gap. The gap size is kept between 0 and
	 * {@link #fThreshold}, leading to re-allocation if needed. The content between
	 * <code>offset</code> and <code>offset + add</code> is undefined after this operation.
	 *
	 * @param offset the offset at which a change happens
	 * @param remove the number of character which are removed or overwritten at <code>offset</code>
	 * @param add the number of character which are inserted or overwriting at <code>offset</code>
	 */
	private void adjustGap(int offset, int remove, int add) {
		final int oldGapSize= gapSize();
		final int newGapSize= oldGapSize - add + remove;
		final boolean reuseArray= 0 <= newGapSize && newGapSize <= fThreshold;

		final int newGapStart= offset + add;
		final int newGapEnd;

		if (reuseArray)
			newGapEnd= moveGap(offset, remove, oldGapSize, newGapSize, newGapStart);
		else
			newGapEnd= reallocate(offset, remove, oldGapSize, newGapSize, newGapStart);

		fGapStart= newGapStart;
		fGapEnd= newGapEnd;
	}

	/**
	 * Moves the gap to <code>newGapStart</code>.
	 *
	 * @param offset the change offset
	 * @param remove the number of removed / overwritten characters
	 * @param oldGapSize the old gap size
	 * @param newGapSize the gap size after the change
	 * @param newGapStart the offset in the array to move the gap to
	 * @return the new gap end
	 * @since 3.3
	 */
	private int moveGap(int offset, int remove, int oldGapSize, int newGapSize, int newGapStart) {
		/*
		 * No re-allocation necessary. The area between the change offset and gap can be copied
		 * in at most one operation. Don't copy parts that will be overwritten anyway.
		 */
		final int newGapEnd= newGapStart + newGapSize;
		if (offset < fGapStart) {
			int afterRemove= offset + remove;
			if (afterRemove < fGapStart) {
				final int betweenSize= fGapStart - afterRemove;
				arrayCopy(afterRemove, fContent, newGapEnd, betweenSize);
			}
			// otherwise, only the gap gets enlarged
		} else {
			final int offsetShifted= offset + oldGapSize;
			final int betweenSize= offsetShifted - fGapEnd; // in the typing case, betweenSize is 0
			arrayCopy(fGapEnd, fContent, fGapStart, betweenSize);
		}
		return newGapEnd;
	}

	/**
	 * Reallocates a new array and copies the data from the previous one.
	 *
	 * @param offset the change offset
	 * @param remove the number of removed / overwritten characters
	 * @param oldGapSize the old gap size
	 * @param newGapSize the gap size after the change if no re-allocation would occur (can be negative)
	 * @param newGapStart the offset in the array to move the gap to
	 * @return the new gap end
	 * @since 3.3
	 */
	private int reallocate(int offset, int remove, final int oldGapSize, int newGapSize, final int newGapStart) {
		// the new content length (without any gap)
		final int newLength= fContent.length - newGapSize;
		// the new array size based on the gap factor
		int newArraySize= (int) (newLength * fSizeMultiplier);
		newGapSize= newArraySize - newLength;

		// bound the gap size within min/max
		if (newGapSize < fMinGapSize) {
			newGapSize= fMinGapSize;
			newArraySize= newLength + newGapSize;
		} else if (newGapSize > fMaxGapSize) {
			newGapSize= fMaxGapSize;
			newArraySize= newLength + newGapSize;
		}

		// the upper threshold is always twice the gapsize
		fThreshold= newGapSize * 2;
		final char[] newContent= allocate(newArraySize);
		final int newGapEnd= newGapStart + newGapSize;

		/*
		 * Re-allocation: The old content can be copied in at most 3 operations to the newly allocated
		 * array. Either one of change offset and the gap may come first.
		 * - unchanged area before the change offset / gap
		 * - area between the change offset and the gap (either one may be first)
		 * - rest area after the change offset / after the gap
		 */
		if (offset < fGapStart) {
			// change comes before gap
			arrayCopy(0, newContent, 0, offset);
			int afterRemove= offset + remove;
			if (afterRemove < fGapStart) {
				// removal is completely before the gap
				final int betweenSize= fGapStart - afterRemove;
				arrayCopy(afterRemove, newContent, newGapEnd, betweenSize);
				final int restSize= fContent.length - fGapEnd;
				arrayCopy(fGapEnd, newContent, newGapEnd + betweenSize, restSize);
			} else {
				// removal encompasses the gap
				afterRemove += oldGapSize;
				final int restSize= fContent.length - afterRemove;
				arrayCopy(afterRemove, newContent, newGapEnd, restSize);
			}
		} else {
			// gap comes before change
			arrayCopy(0, newContent, 0, fGapStart);
			final int offsetShifted= offset + oldGapSize;
			final int betweenSize= offsetShifted - fGapEnd;
			arrayCopy(fGapEnd, newContent, fGapStart, betweenSize);
			final int afterRemove= offsetShifted + remove;
			final int restSize= fContent.length - afterRemove;
			arrayCopy(afterRemove, newContent, newGapEnd, restSize);
		}

		fContent= newContent;
		return newGapEnd;
	}

	/**
	 * Allocates a new <code>char[size]</code>.
	 *
	 * @param size the length of the new array.
	 * @return a newly allocated char array
	 * @since 3.3
	 */
	private char[] allocate(int size) {
	    return new char[size];
    }

	/*
	 * Executes System.arraycopy if length != 0. A length < 0 cannot happen -> don't hide coding
	 * errors by checking for negative lengths.
	 * @since 3.3
	 */
	private void arrayCopy(int srcPos, char[] dest, int destPos, int length) {
		if (length != 0)
			System.arraycopy(fContent, srcPos, dest, destPos, length);
	}

	/**
	 * Returns the gap size.
	 *
	 * @return the gap size
	 * @since 3.3
	 */
	private int gapSize() {
		return fGapEnd - fGapStart;
	}

	/**
	 * Returns a copy of the content of this text store.
	 * For internal use only.
	 *
	 * @return a copy of the content of this text store
	 */
	protected String getContentAsString() {
		return new String(fContent);
	}

	/**
	 * Returns the start index of the gap managed by this text store.
	 * For internal use only.
	 *
	 * @return the start index of the gap managed by this text store
	 */
	protected int getGapStartIndex() {
		return fGapStart;
	}

	/**
	 * Returns the end index of the gap managed by this text store.
	 * For internal use only.
	 *
	 * @return the end index of the gap managed by this text store
	 */
	protected int getGapEndIndex() {
		return fGapEnd;
	}
}
